Marconi TF2700 Bridge - UK Vintage Radio Repair and Restoration Discussion Forum

Wendymott · 23rd Nov 2020, 4:40 pm

Hi Peeps....
I hope you learned members may shed some light on a problem I have come across.
I needed some 220uH RF chokes....... as I had in stock some of the mini axial types that are obviously Ferrite based, I decided to check the values on my Marconi bridge, where I got some very variable results, as in measuring well below that value. And the bridge was not balancing all that well. Putting the same inductors on my Electronic LCR meter, it gives a very near 220uH reading.
I suspect the Marconi was designed well before these modern components, but I would have expected some reasonable correlation of measurement.
The Marconi measuring frequency is 1 Khz.....The Chinese meter has 3 options 100 Hz 1 Khz and 10Khz.... all with similar readings.

Ed_Dinning · 23rd Nov 2020, 10:05 pm

Hi Wendy, low value inductors, especially with low resistance can be difficult to measure on these Marconi bridges. Correct adjustment of the loss balance is essential or you can wind up chasing a null down to zero.
It is often a 2 handed operation to get balance

Ed

Dickie · 23rd Nov 2020, 11:04 pm

I've just tried a 470uH choke on my TF2700 and it produced a nice sharp null at 440uH and also managed to get a resistance reading of 25R, but the null was a bit more indistinct. So maybe your 2700 is faulty?

Wendymott · 24th Nov 2020, 12:27 am

Hi Ed..... I always use two hands with the TF2700........ Dickie.... It does "null" with larger values as 470nH..... but down at the 220uH it is very difficult to see a dip.
If it was faulty I would expect all sorts of errors.. not just on inductors...maybe I should measure small inductors with my other test jig which is a Vakar osc .. frequency counter and calculator. I use this on SMD inductors.

Diabolical Artificer · 24th Nov 2020, 7:54 am

Yep, mine is tricky to use too on unknown L's and C's as well as tricky down the bottom end, there's an art to using them.

Andy.

Wendymott · 24th Nov 2020, 10:59 am

I used to use the "other" Marconi bridge when I worked at Bradford uni...the one with the sloping front.... sorry I dont know the model number..but I do like the TF2700, its small.ish to fit in the cupboard ...and it works well on the mid to larger values.

barrymagrec · 24th Nov 2020, 12:34 pm

Tf1313a.

23rd Nov 2020, 4:40 pm	#1
Wendymott Octode Join Date: Jun 2013 Location: Huddersfield, West Yorkshire, UK. Posts: 1,795	Marconi TF2700 Bridge Hi Peeps.... I hope you learned members may shed some light on a problem I have come across. I needed some 220uH RF chokes....... as I had in stock some of the mini axial types that are obviously Ferrite based, I decided to check the values on my Marconi bridge, where I got some very variable results, as in measuring well below that value. And the bridge was not balancing all that well. Putting the same inductors on my Electronic LCR meter, it gives a very near 220uH reading. I suspect the Marconi was designed well before these modern components, but I would have expected some reasonable correlation of measurement. The Marconi measuring frequency is 1 Khz.....The Chinese meter has 3 options 100 Hz 1 Khz and 10Khz.... all with similar readings. __________________ Should get out more. Regards Wendy G8BZY

23rd Nov 2020, 10:05 pm	#2
Ed_Dinning Dekatron Join Date: Jan 2004 Location: Newcastle upon Tyne, Tyne & Wear, UK. Posts: 8,195	Re: Marconi TF2700 Bridge Hi Wendy, low value inductors, especially with low resistance can be difficult to measure on these Marconi bridges. Correct adjustment of the loss balance is essential or you can wind up chasing a null down to zero. It is often a 2 handed operation to get balance Ed

23rd Nov 2020, 11:04 pm	#3
Dickie Octode Join Date: Sep 2004 Location: St. Albans, Hertfordshire, UK. Posts: 1,478	Re: Marconi TF2700 Bridge I've just tried a 470uH choke on my TF2700 and it produced a nice sharp null at 440uH and also managed to get a resistance reading of 25R, but the null was a bit more indistinct. So maybe your 2700 is faulty? __________________ Regards, Richard, BVWS member

24th Nov 2020, 12:27 am	#4
Wendymott Octode Join Date: Jun 2013 Location: Huddersfield, West Yorkshire, UK. Posts: 1,795	Re: Marconi TF2700 Bridge Hi Ed..... I always use two hands with the TF2700........ Dickie.... It does "null" with larger values as 470nH..... but down at the 220uH it is very difficult to see a dip. If it was faulty I would expect all sorts of errors.. not just on inductors...maybe I should measure small inductors with my other test jig which is a Vakar osc .. frequency counter and calculator. I use this on SMD inductors. __________________ Should get out more. Regards Wendy G8BZY

24th Nov 2020, 7:54 am	#5
Diabolical Artificer Dekatron Join Date: Jul 2009 Location: Sleaford, Lincs. UK. Posts: 7,667	Re: Marconi TF2700 Bridge Yep, mine is tricky to use too on unknown L's and C's as well as tricky down the bottom end, there's an art to using them. Andy. __________________ Curiosity hasn't killed this cat...so far.

24th Nov 2020, 10:59 am	#6
Wendymott Octode Join Date: Jun 2013 Location: Huddersfield, West Yorkshire, UK. Posts: 1,795	Re: Marconi TF2700 Bridge I used to use the "other" Marconi bridge when I worked at Bradford uni...the one with the sloping front.... sorry I dont know the model number..but I do like the TF2700, its small.ish to fit in the cupboard ...and it works well on the mid to larger values. __________________ Should get out more. Regards Wendy G8BZY

24th Nov 2020, 12:34 pm	#7
barrymagrec Octode Join Date: Dec 2017 Location: Morden, Surrey, UK. Posts: 1,559	Re: Marconi TF2700 Bridge Tf1313a.